1. Field of the Invention
This invention relates to a drive system for a DC motor, and more particularly, to a drive system for a DC motor with reduced power loss.
2. Description of the Prior Art
In a conventional system, when controlling the rotational speed of a DC motor, a drive transistor is used to supply the DC motor with a controlled voltage from a DC voltage source. The drive transistor controls the electrical power to the DC motor by changing the voltage drop between the collector and emitter terminals. In this case, the supplied power from the DC voltage source is the sum of the effective power in the DC motor and the power loss (the collector loss) of the drive transistor. The power loss of the drive transistor is generally larger than the effective power in the DC motor.
Accordingly, the power efficiency, defined as the percent ratio of the effective power supplied to the DC motor to the total power supplied from the DC voltage source, is rather small. Particularly, in a control system for a DC motor having a wide range of rotational speeds or output torque, the power efficiency becomes remarkably small at low speed or low torque. (For example, a capstan motor for an audio or video cassette tape recorder has to rotate in a wide range of rotational speeds from low speed in its play mode to high speed in its cue mode, and its power efficiency is about 20% in its play mode.)
U.S. Pat. No. 4,359,674 discloses a control system for a DC motor having a DC-DC switching converter in order to improve the power efficiency. When such a control system for a DC motor is used for controlling the rotational speed, the current to the polyphase coils at the starting or acceralating period is large enough to acceralate the DC motor rapidly. But the current to the polyphase coils at the controlled period becomes small corresponding to a small load torque. For example, the current to the polyphase coils is 2 A at the starting period but 250 mA at the controlled period.
The DC-DC switching converter supplies the polyphase coils with the above current through a switching transistor. So, the base current of the switching transistor has to be large enough to deal with the current 2 A at the starting period.
Assuming that current gain h.sub.FE of the switching transistor is 50 and that overdrive factor of the switching transistor is 2, the necessary base current of the switching transistor is at least (2 A/50).times.2=80 mA at the starting period. The overdrive factor is defined as ratio of the actual base current of the switching transistor to the minimum base current I.sub.c /h.sub.FE to flow the collector current I.sub.c. On the other hand, the necessary base current of the switching transistor at the controlled period is only (250 mA/50).times.2=10 mA corresponding to the current 250 mA.
Thus, the excess base current, 80 mA-10 mA=70 mA, causes a power loss, which decreases the power efficiency. Assuming that the voltage of the DC voltage source and the output voltage of the DC-DC switching converter are 20 V and 7 V, respectively, the power loss is about 20 V.times.70 mA.times.0.35=0.49 W, where 0.35 is the ON-time ratio of the switching transistor, defined as ratio of ON-time to one cycle time.
The power supplied to the DC motor is 7 V.times.250 mA=1.75 W, thus the power loss 0.49 W of the excess base current is equal to 28% of the effective power 1.75 W. As the DC motor usually rotates at the controlled rotational speed, the above power loss has to be reduced for improving the power efficiency.